1. Field of the Invention
The present invention relates to an optical displacement sensor for optically measuring a displacement amount of an object.
2. Description of the Related Art
Conventionally, a Michelson interferometer such as shown in FIG. 20, is known as an optical mechanism for measuring a displacement amount of an object by using a semiconductor sensor.
More specifically, a laser beam emitted from a semiconductor laser 1 is shaped into a parallel beam by a collimator lens 3, and then the parallel beam is irradiated via an optical isolator 5 on a beam splitter 7, where the beam is divided in two directions.
One of the split laser beams reflected from the beam splitter 7 is irradiated on a fixed mirror 9, by which the beam is reflected back towards the beam splitter 7. The other laser beam permeates the beam splitter 7, and reflects on a mirror 11 mounted on an object (not shown). After that, this beam also reflects back towards the beam splitter 7.
The laser beams reflected on the mirrors 9 and 11 are combined back together by the beam splitter 7, and the combined beam is converged on a light reception element 15 via a converging lens 13.
At the light reception element 15, the variation state (for example, the variation state of the dark/bright contrast of an interference stripe) of the light intensity which corresponds to the difference between the two beams in optical path, is detected.
The displacement amount (S) of the mirror 11 mounted on the object is measured based on the data detected by the light reception element 15.
An optical isolator 5 has a function of shutting off the light which is returning to the semiconductor laser 1, in order to prevent problems including an oscillation mode change (mode hopping) of the semiconductor laser 1, a leap of an oscillation wavelength and a variation of the laser output.
Meanwhile, a method of measuring a displacement of an object to be measured, by using a complex resonator having a structure in which a laser beam is irradiated from the semiconductor laser 1 to the object and the reflection light from the object is sent back to the semiconductor laser 1 has been proposed (see Jap. Pat. Appln. KOKAI Publication No. 60-256079), and the structure thereof is shown in FIG. 21.
As shown in FIG. 21, a laser beam emitted from the semiconductor laser 1 is converted into a parallel beam by the collimate lens 3, and then divided into two direction normally crossing with each other by the beam splitter 7.
One of the split beams is irradiated vertically on the external mirror 11 mounted on the object, and the reflection light returns to the semiconductor laser 1 via the common path as of the emitted light beam. The other laser beam is irradiated on an optical detector 5, where the optical output thereof is measured.
Supposing that the external mirror 11 is displaced in an X or -X direction, the displacement of the external mirror 11 can be measured based on the phase relationship between an emission light of the semiconductor laser 1 and its reflection light. In detail, each time the displacement of the external mirror 11 changes by a half of the oscillation wavelength .lambda..sub.0 (that is, .lambda..sub.0 /2), the intensity of the optical output changes. By utilizing such a phenomenon, the displacement of the external mirror 11 can be measured.
However, since an interferometer such as shown in FIG. 20, includes a lens 13, an optical isolator 5, a beam splitter 7, mirrors 9 and 11, and the like, which are arranged in combination to divide or synthesize a laser beam. Therefore, it requires a large volume to assemble a whole optical system, and therefore it is difficult to integrate those members together as a very small micro sensor.
The structure shown in FIG. 21 necessitates a collimation lens 3 for collimating a laser beam, and a beam splitter 7, and therefore the downsizing of the sensor is limited.
Further, in the case where a regular semiconductor laser having a stripe structure is used in the structure shown in FIG. 21, the oscillation mode in the semiconductor laser changes (mode hopping) as the amount or phase of the light returning to the semiconductor laser changes, and therefore the variation of the optical output is likely to occur. Accordingly, a regular change in optical output in every .eta..sub.0 cycle with respect to a displacement x and a change in optical output due to irregular mode hopping, overlap with each other, and an accurate measurement of the displacement amount of the object cannot be carried out.
Further, U.S. Pat. No. 5,331,658 discloses a sensor which uses a surface-emitting laser such as shown in FIG. 22. In this document, reflection means 45 is provided above an upper mirror 24 of the surface-emitting laser (SEL) with a displacement of a 1/4 or 1/2 wavelength therebetween, and the sensor operates in the following manner. By utilizing a change in the threshold current of SEL due to a deformation of reflection means 45, a change in an environment such as pressure or temperature can be detected.
In this prior art document, however, the reflection means 45 is integrally provided on the surface-emitting laser (SEL), and is located at a position only a 1/2 (or 1/4) wavelength away from the upper mirror 24 of the surface-emitting laser (SEL).
Further, this prior art document does not disclose how the sensor operates in the case where the displacement between the external reflection means 45 and the upper mirror of the surface-emitting laser is larger than the 1/2 wavelength.
Therefore, not only a periodical change in laser output caused by a positional change of the means when the reflection means is located a 1/2 wavelength or more away is not exhibited, but also the detection range is limited to a maximum of 1/2 wavelength. Therefore, the detection range is narrowed (for a wavelength of laser of 1000 nm, the range is 500 nm-0.5 .mu.m). Further, it is very difficult to build the reflection means accurately at a position a 1/2 (or 1/4) wavelength away from the surface.